Reservoirs that have become known up to the present include an electro-optical sensor comprising an emitter and a receiver disposed on opposite sidewalls of the reservoirs on an optical axis extending across the full cross-section of the reservoir generally parallel to the bottom thereof, i.e. the emitter and the receiver are located at the same level above the bottom of the reservoir. This results in a relatively great length of the optical axis, whereby the adjustment or alignment of the emitter and receiver is rendered rather difficult. If a reservoir of this type is moreover provided with conveyor means, for instance a screw conveyor for conveying the coffee meal from the reservoir to the brewing section, the optical axis cannot be disposed as closely over the bottom as would in fact be required for accurately determining a minimum filling level.